A disproportionately large amount of the hydrocarbons produced by a vehicle are emitted during cold starting of the vehicle engine. The temperature of the intake passages and the combustion chambers of the engine during the early stages of a cold start inhibit the proper vaporization of fuel. As a result, during cold start, a stoichiometric air/fuel ratio is difficult to achieve with intake port fuel injection. In addition, unburned fuel vapor is delivered to the catalytic converter together with the normal by-products of combustion. During the early stages of a cold start, the catalyst material in the catalytic converter has not reached a sufficient temperature in order to sufficiently process the unwanted, and uncombusted, by-products of combustion. Tailpipe emissions of hydrocarbons thus increase as a result of all three of the foregoing factors.
One solution to reducing hydrocarbon emissions during a cold start is the use of an Electrically Heated Catalyst (EHC). The EHC employs resistive elements which heat the catalyst prior to starting the engine. In cold start, the heated catalyst is thus better able to process the unwanted by-products of combustion. Unfortunately, use of the EHC adds additional cost, extra complexity, and requires a delay prior to engine starting to allow the EHC to preheat the catalyst.
The inventors herein have recognized that direct fuel injection may be used to advantage in reducing hydrocarbon emissions during cold start. Direct fuel injection offers control of fuel delivery unachieveable with intake port fuel injection, where the fuel injector is positioned outside of the combustion chamber. Others have utilized direct fuel injection to achieve engine control strategies unachievable in engines utilizing intake port fuel injection. For example, Sasaki et al. in U.S. Pat. No. 5,207,058 entitled Internal Combustion Engine describe an engine which utilizes direct fuel injection and which employs a control strategy to raise the temperature of the catalytic converter if it is found to be below a predetermined minimum temperature. However, Sasaki et al. do not contemplate reduction of hydrocarbon emissions by achieving immediate combustion as quickly as possible upon cold start. Moreover, Sasaki et al. appear to contemplate an engine which has achieved stable combustion. Thus, the approach contemplated by Sasaki et al. has limited use in cold start of an engine.